I had a similar problem and it turned out that the points were sticking and we also adjusted the carburetor for good measure. In your case I would also considor a fuel problem. Since your problem gets worse after you are underway I'm thinking that perhaps you have a lot of sludge in the bottom of your tank and when you are underway it gets sloshed around. How long has it been since you changed your fuel? I mean really changed it and not just added to it. remember that adding new fuel to old fuel makes the new fuel worse and doesn't make the bad fuel better. After years of topping off your fuel tank you can end up with a tank of contaminated fuel. After checking the points I would siphon all of your fuel out and start over with a fresh tank. Make sure you siphon from the bottom so you get any contaminates that have settled. I would also completely clean/rebuild your carburetor at the same time. Hope this helps.